Oxygen gas sensors are known for use in measuring the oxygen content of an exhaust gas, such as in automobiles, to regulate the efficiency of an engine by varying the air to fuel ratio. One type of such an oxygen gas sensor has a solid electrolyte body in the general shape of a thimble, the solid electrolyte generally comprising a stabilized zirconium dioxide material, with electrodes formed on both the interior and exterior surfaces of the sensor element, the electrodes normally being formed of a catalytic material such as platinum. With the inner electrode exposed to a reference gas such as air, and the outer electrode exposed to the exhaust gas, the oxygen content of the exhaust gas can be measured to determine and regulate the air-fuel ratio of the gas mixture inlet to the engine. The voltage output of such zirconium dioxide sensors normally produce a voltage output the magnitude of which depends upon the oxygen partial pressure in the exhaust gas in which the sensor is immersed. Typically, such sensors, at exhaust temperatures above 350.degree. C., should produce a voltage of about 900 millivolts in an exhaust gas richer than stoichiometry and about 50 millivolts in an exhaust gas leaner than stoichiometry. It has been found, however, that instead of the supposed 900 millivolts to 50 millivolts range, the output ranges of such sensors often would be as low as 0-400 millivolts in the rich gas phase and a negative 200-600 millivolts in the lean phase. The negative voltages occurred most frequently during low exhaust gas temperatures (350.degree. C. or below) and tended to fall into more negative range with increased use.
In addition, the switching time or time required for the sensor to detect a change from rich to lean or lean to rich exhaust gas compositions must be as low as possible, preferably below about a half second (500 milliseconds), again especially during low temperature operation (about 350.degree. C.) such as during engine warm-up.
The internal resistance of the sensor is a further factor which must be controlled since a low internal resistance will allow the sensor to sink or source more useful current from the monitoring system that is being used for determining the oxygen content of the exhaust gas.
The present process provides for the preparation of an oxygen gas sensor element which evidences these improved properties, a high positive voltage output, a fast switching time response and a low internal resistance.